Marine vessels use various propulsion systems or units. The main propulsion unit or units is/are normally arranged in the aft part of the ship. The main propulsion unit may be either a fixed propeller arrangement creating a thrust force in the longitudinal direction of the marine vessel, or it may be a pod or a thruster, i.e. a propeller arrangement that may be rotated round a vertical axis.
The marine vessels have also other propulsion arrangements that are mainly used when manoeuvring a ship in a port, for instance. One type of such propulsion arrangements is a tunnel thruster, which may be used both at the bow and at the stern of a ship. The tunnel thruster is arranged in a horizontal tunnel running transverse to the longitudinal direction of the marine vessel through the hull of the marine vessel for assisting in moving the entire ship or one end of the ship sideways for instance for docking purposes.
The tunnel thrusters have been developed further by making such retractable i.e. the thruster unit may be kept within the hull but it may also be lowered below the hull i.e. below the baseline of the marine vessel. When the thrusters unit is in lowered position it may be rotated round a vertical axis, and it may thus be used to generate thrust in any desired direction for steering purposes.
As an example of a prior art retractable thruster U.S. Pat. No. 3,550,547 may be discussed. The document discloses an auxiliary maneuvering means for ships, which provide an improved maneuvering capability in deep water and which also are applicable when travelling in low waters, wherein a lateral thrust device is combined with a slewing propeller. The centre part of the tunnel of the lateral thrust device together with the propeller arranged therein is located in a recess in the interior of the ship. The centre part of the tunnel is constructed as nozzle, which is downwardly extendible and in the extended position turnable around 360°.
As another example of a prior art retractable thruster U.S. Pat. No. 5,522,335 may be discussed. The document discloses an auxiliary thruster for a marine vessel. The auxiliary thruster includes a submersible propulsion unit which has a shroud with a propeller rotatably mounted therein. A canned electric motor is mounted between the propeller and the shroud for rotating the propeller to create thrust. A propulsion unit deploying and rotating mechanism is mounted on the hull and on the propulsion unit. The propulsion unit deploying and rotating mechanism is operable to extend the propulsion unit out of the hull and retract it into the hull and to rotate the propulsion unit to direct the thrust generated thereby in any desired direction when the thruster is in the deployed position. When the thruster is retracted, it is positioned with a tunnel extending transversely through the hull. Rotation of the propeller while in the retracted position generates laterally directed thrust through the tunnel.
Both above discussed documents illustrate a retractable tunnel thruster where the thruster propeller is lowered with a part of the tunnel. In other words, with the propeller a lengthy conical or cylindrical tunnel part is moved below the keel of the marine vessel. The length of the tunnel part is of the order of the diameter of the propeller or even more, i.e. long enough to house the thruster entirely. When operating below the keel such a long tunnel part increases the flow resistance of the water ‘pumped’ through the tunnel part as well as outside the tunnel part. Also, the force needed to turn the thruster unit in its lowered position is relatively big, as the retractable tunnel part is long.
An object of the present invention is to optimize the structure of the retractable tunnel part for minimizing the flow resistance.